The vast majority of colour electronic displays, including transmissive (such as liquid crystal display (LCD)), emissive (such as organic light-emitting diode (OLED)) and reflective (such as electrophoretic) types, use three or more different colour type sub-pixels within each display pixel, in order to be able to show a composite colour from each pixel unit with a high degree of control of both chromaticity and luminance. This is achieved by independently modulating the amount of light transmitted, reflected or emitted by each of the colour-type sub-pixels comprising a pixel to produce the intended additive colour mixture. In the most common display type, a transmissive LCD, each composite display pixel comprises a red (R), green (G) and blue (B) sub-pixel. These sub-pixels are arranged in a vertical stripe pattern, each sub-pixel having an area of approximately one-third of the composite pixel area, and a rectangular shape with 3:1 aspect ratio, so that the composite pixel has a square geometry providing equal image resolution (both black/white and colour resolution) in the horizontal and vertical directions. (See, e.g., FIG. 1(a)).
Several types of display devices are known which utilise different areas and/or shapes for the different sub-pixels. Prominent examples include the “Pentile RGBG” and “Samsung RGBG” pixel layouts used in OLED displays, in which the green sub-pixels are smaller than the R and B sub-pixels in all the pixels. (See, e.g., FIGS. 1(b) and 1(d)). This is done for reasons of equalising the emissive lifetime of the different electroluminescent materials used in the different colour types. These display types also may have sub-pixels of different colour types within each composite pixel, each composite pixel comprising a G sub-pixel and either an R or B sub-pixel. This is done for reasons of exploiting the human visual system's (HVS) reduced acuity at red and blue wavelengths to minimise the total number of sub-pixels required to faithfully display an image. These layouts are disclosed in U.S. Pat. No. 6,867,549 B2 (Cok et al., issued Mar. 15, 2005) and U.S. Pat. No. 8,354,789 B2 (Gun-Shik et al., issued Jan. 15, 2013).
As image data content in all common formats is almost exclusively configured for display on devices with R, G and B sub-pixels, and therefore assumes each pixel of the display is capable of displaying any mixture of R, G and B, including black and white, displays of this type are usually configured with a built-in image processing function to reconfigure the input data for display on the particular device. This enables the display to optimally transfer luminance intended for the red sub-pixel of a pixel, for example, from a composite pixel which has no red sub-pixel, to a neighbouring one which does, with minimal impact on the perceived image quality in terms of sharpness or colour fidelity (U.S. Pat. No. 8,817,056 B2, Jong-Woong et al., issued Aug. 26, 2014).
Similarly, pixel layouts using multiple sub-pixels of one or more colour type, in conjunction with only one of another colour type, to better match the HVS colour resolution, or allow improved image appearance via sub-pixel rendering methods, have been disclosed in U.S. Pat. No. 7,646,398 B2 (Brown Elliot, issued Jan. 12, 2010).
Displays in which each composite pixel comprises one of each type of colour sub-pixel utilised in the display, but in which the different colour type sub-pixels within the composite pixel are provided with different relative areas are known. Examples include Sharp Electronics Corporation's Quattron™ RGBY type display, in which each pixel has four sub-pixels, the G and Y type sub-pixel having a smaller area than the R and B types. (See, e.g., FIG. 1(c)). This is done for reasons of maintaining a balanced white colour when all the sub-pixels are made fully transmissive.
Displays having pixels of only a single colour type (e.g. transparent), for use with time sequential coloured backlight illumination, in which the sub-pixels within a pixel are different sized, so as to enable temporal and spatial dither of pixels which are only capable of a binary transmission control, are known, for example, U.S. Pat. No. 5,905,482 (Hughes et al., issued May 18, 1999).
However, all of these methods either have only one type of colour sub-pixel with binary transmission control, or retain a fixed area for all the sub-pixels of a given type within the display. Therefore, while they are able to trade-off resolution in a particular colour channel for a reduced number of pixels overall, or they may optimally utilise the available display area to provide the intended white hue when all pixels are maximally bright, they are not able to trade off resolution within a single colour channel for increased bit-depth, improved wide-view performance, response time or other display metrics which may be improved by providing an increased number of average luminance gradations from a group of same colour-type but differing area sub-pixels, using a fixed number of voltage or current addressing gradations. Nor are they able to provide multiple configurations of pixel luminances within a group of same colour-type but differing area sub-pixels, while maintaining a fixed overall luminance, thereby allowing sub-pixel rendering within a single colour channel, improved wide-view performance, or other display metrics.